Among energy storage devices such as a lithium ion secondary battery, there has been known an energy storage device where an electrode assembly which includes a positive electrode and a negative electrode is connected to electrode terminals through current collecting members. For example, JP 2010-212240 A discloses a secondary battery which is configured such that insulators which form insulating members and a cap plate which forms a lid body of a container are sandwiched between current collecting members and electrode terminals connected to each other. The first insulator is disposed between the electrode terminal and the cap plate, and the second insulator is disposed between the cap plate and the current collecting member. A shaft of the electrode terminal penetrates the insulator and the cap plate, and is connected to the current collecting member. An extension groove is formed on the second insulator, and an extension portion is formed on the current collecting member. The extension groove and the extension portion are configured to be joined to each other.
In the secondary battery described in JP 2010-212240 A, due to joining between the extension groove and the extension portion, the relative rotation between the second insulator and the current collecting member about the shaft of the electrode terminal can be prevented. The extension groove and the extension portion are disposed at a position closer to an end portion of the cap plate than the shaft of the electrode terminal is. To be more specific, the extension portion is disposed at an end portion of the current collecting member in the vicinity of the end portion of the cap plate, and the extension groove is disposed between the extension portion and the end portion of the cap plate. Accordingly, it is necessary to ensure a space between the end portion of the current collecting member and the end portion of the cap plate. Thus, the electrode assembly for storing energy cannot be disposed in the space. A volume of the electrode assembly in a container of the secondary battery is forced to be small and hence, energy density of the energy storage device is lowered.